1. Field of the Invention
The present invention relates to a driving device for a piezoelectric element which, in the case of the use of the piezoelectric element as an electromechanical converter, electrically drives the piezoelectric element to obtain a predetermined mechanical output. The present invention more particularly relates to a driving device for a piezoelectric element suitable for use as a piezoelectric actuator, such as for performing reciprocal motion, to thereby provide the mechanical output for a unit injector for supplying fuel to an internal combustion engine, for a hydraulic valve, etc.
2. Description of the Related Art
In the prior art, wide use has been made of rotary electrical machinery, electromagnets, etc. exhibiting electromagnetic phenomena as electromechanical converters. The increased use of electronics for mechanical systems, which has arisen along with the greater sophistication of mechanical systems in recent years, has led to strong demands for higher speeds in solenoid valves and other actuators combining electromagnets and hydraulic circuits.
In accordance with this, there have been proposed various actuators which use piezoelectric elements. Such actuators hold forth the promise of extremely high speed operation free from the problems of eddy current and magnetic energy influx, which limit speeds in the case of application of electromagnetic phenomena. For example, Japanese Unexamined Patent Publication (Kokai) No. 59-77043 discloses an example of the use of the high speed response and high power characteristics of piezoelectric elements for a unit injector.
However, in this type of device, in general a high voltage and large current are demanded as the electrical drive conditions. Therefore, the DC power source becomes large, and caution is required as to the use of high voltage, thus creating a substantial obstacle to practical use. Further, the injection quantity of the unit injector of the above prior art device is changed along with displacement of the piezoelectric element. This displacement is in general controlled by the amount of voltage applied. The amount of displacement, however, has a large temperature coefficient, so there is the problem that precise control of the injection quantity is difficult.